1. Field of the Invention
The present invention relates to a color filter fabrication in a TFT-LCD process, and in particular to a method to remove color resist above alignment marks for photolithography alignment.
2. Description of the Related Art
The size of TFT-LCD display area is increased due to market demand and fabrication improvement. The size of base materials, such as glass substrates, is also required to be enlarged correspondingly. For example, the size of the glass substrates was 300 mm×400 mm in 1989, 500 mm×650 mm in 1996 and is now 1000 mm×1000 mm in a fifth generation.
The TFT-array and color-filter substrates are made into an LCD panel by assembling the two substrates together with a sealant, while the cell gap is maintained by spacers. Contrary to the transparent photoresist (i.e. a positive photoresist) used in TFT-array fabrication, the color filter is fabricated by color-resists, i.e. the primary colors of red, blue and green, formation and definition. For small size color filter, the conventional photolithography of a color filter substrate covered with a color resist thereon is conducted by one shot exposure with a mask that is approximately equal to the color filter substrate in dimension. There are usually alignment marks pre-disposed at the edges of the color filter substrate for a photolithography alignment to calibrate the exposure location. Two issues arise in the above process. First, the color resist covering the alignment marks results in difficulty in alignment due to the low transparency of the color resist. The low contrary and high optical density of the color resist also cause difficulty in alignment. Second, the color resist layer at the edge of the substrate is thicker than in the center, which results in a different developing rate for the color resist layer. The unevenness of the color resist layer leads to color resist remaining and thus contaminating subsequent fabrication equipment. A conventional solution for uneven color resist deposition is to remove the color resist layer from the edge before exposure by Edge Beam Removing (herein EBR) to expose the alignment marks. Specific solvents are utilized for EBR equipment to dissolve the overly thick color resist from the edge and then wash away the waste color resist solution.
FIGS. 1A and 1B further illustrate a conventional process of color resist removal from the edge of a color filter substrate.
A color filter substrate 10 is provided with alignment marks 12 disposed thereon for alignment and a color resist layer 14 formed over the color filter substrate 10. An EBR equipped with four EBR heads 16 is utilized to remove the color resist layer from the edge of the substrate 10. The movement 17 of the four EBR heads 16 along the four sides of the rectangular substrate 10 is shown in FIG. 1A. The EBR heads 14 inject a specific solvent 18 at the edge of the substrate 10 to dissolve the color resist thereon, as shown in FIG. 1B. An exhaust 20 on the EBR head 14 extracts the dissolved color resist and the remaining solvent, consequently exposing the alignment marks 12 on the substrate 10 and preventing color resist residue after exposure.
For fabricating large size color filters, such as substrates over 1000 mm×1000 mm, masks for photolithography are smaller than color filter substrates, which means multi-shot exposure for the whole substrate is required. The exposure of a large size color filter is usually performed by multi-exposing. For example, a 1000 mm×1000 mm color filter substrate is exposed with a stepper for four alignments and four exposures. To achieve this, alignment marks have to be pre-disposed on both the central area and the edge of the substrate to align the stepper. However, conventional EBR can only remove the color resist from the edge but not from over the alignment marks on the central substrate. The low transparency of the color resist on the central substrate blocks the alignment marks and results in difficulty in stepper alignment.
The conventional solution is to modify the alignment modules on exposure equipment or to improve the transparence characteristics of color resist. However, these solutions usually create higher costs and provide only limited improvement.